I usually bring a small soft cooler on wheels as my carry-on. I pack it like a suitcase, but use it like a cooler once at the resort. It can also make taking cold drinks to the pool much easier.
I also like to bring a pitcher, cold brew iced tea, liquid splenda, and a funnel. I'll stay much more hydrated if I'm not drinking plain water, and the cold brew is so easy to make. I use the funnel to make it easy to pour the tea into water bottles to take into the parks. You can freeze them overnight, too, so it'll stay cold, longer. It's also way cheaper than those crystal light individual serving packages.
An extra pair of glasses, in case mine break.
Straws and 2 large plastic cups, so I can make mixed drinks for hubby and me (those tiny plastic cups don't hold nearly enough for us!).
Ductape - it'll fix nearly anything. I've patched torn clothes with this (put it on the inside and it won't show much at all, and will even last through a couple washings!) and a busted suitcase.
Dishsoap, for washing our cups and reusable water bottles.
Fork, spoon, and knife for eating leftovers.
Booklight so I don't keep hubby awake.